Okay since Foul Play does more damage the opponent's attack stat is higher than yours....If I came in with a Level 1 Sableye, intimidated by a gyrados, and the gyarados was +6 attack, adamant, 252 attack evs, and I used foul play, how much would it do? What mattered?

If a generic Modest (competitive wifi) Alakazam uses Foul Play on a Adamant (252 attack+competitive wifi) Reggigas, will it do tons of damage?

The reason for the Sableye one doing such a minuscule amount of damage is because it's level 1. Level plays a role in damage calculation, so despite Gyarados and thus Sableye having extremely high Attack, Sableye's level makes its attacks extremely weak. If we were to put is level back to 100, it'd do around 250% of Gyarados' HP, well over the OHKO.

For the Alakazam one, it doesn't do much since it isn't STAB, it has no item and also partially because Regigigas is naturally bulky with base 110 Defence.